T lymphocytes initiate and regulate the complex immune reaction of the organism to foreign antigens. The T cell response begins when antigen receptors on the surface of T cells engage antigen in the context of surface molecules encoded by the major histocompatability complex. This antigen receptor is a multicomponent cell surface complex consisting of as many as nine chains. Receptor occupancy leads to rapid activation of two protein kinase pathways and subsequent phosphorylation of the receptor and other cellular substrates. The approach of the laboratory has been completely characterize the receptor on a murine T cell hybridoma with antibodies binding the component chains. Work has been extended to biochemical analysis of receptors on normal human and murine T cells. Characterization of protein kinase activation and regulation, especially activation of a protein tyrosine kinase has been extensively studied in order to understand signal transduction in normal T cells and those isolated from murine and human disease states.